The present invention relates to an overhead projector sheet (hereinafter abbreviated as xe2x80x9can OHP sheetxe2x80x9d) for forming images thereon by the use of an electrophotographic recording system.
In recent years, full-color copiers utilizing an electrophotographic recording system have been placed on the market by various manufacturers of electric appliances. Very recently full-color electrophotographic copying technology has made a remarkable progress, and copiers capable of providing recorded images of near-graphic arts quality or near-photographic print quality have come to appear on the market.
However, on viewing electrophotographic OHP sheets used in full-color copiers on the electrophotographic recording system, we observe that problems still remain to be solved. The particularly serious problem consists in that the OHP sheets after image formation have a very greasy touch to leave unpleasant feeling to the users"" fingers. This problem is caused by transfer of oil such as silicone oil, which is applied to a fixing roller at the time of toner fixation, onto the OHP sheet surface.
Further, as the images recorded on OHP sheets are projected on a screen through an overhead projector (hereinafter abbreviated as xe2x80x9cOHPxe2x80x9d), the OHP sheet are required to ensure not only excellent color reproducibility but also high transparency in the recorded images.
In the case of recording images on electrophotographic OHP sheets by means of a full-color copiers on the electrophotographic recording system, the thermal fixation of toner images has so far been carried out at a lower speed (referred to as xe2x80x9cOHP modexe2x80x9d) than the fixation speed for copying paper (referred to as xe2x80x9cPAPER modexe2x80x9d) for the purpose of increasing the toner fixability and enhancing the transparency of recorded images. When the PAPER mode is actually used for recording images on commercially available electrophotographic OHP sheets, the images recorded are inferior in transparency, and so they cannot ensure good lightness, saturation and color reproducibility in the images projected on a screen to result in projection of brackish gray images. Therefore, it has so far been desired to develop electrophotographic OHP sheets which can provide images having excellent color reproducibility and high transparency even when the recording is carried out at a higher fixing speed (in PAPER mode).
In order to solve the aforementioned problems, various manufacturers of electric appliances have developed electrophotographic recording system-utilized copiers enabling complete or considerable elimination of oil supply to fixing rollers (heat rollers) at the time of toner fixation and thereby leaving no oily touch on the OHP sheet surface, and further enabling high-speed recording through thermal treatment with high-temperature fixing rollers.
However, when conventional electrophotographic OHP sheets are used in those newly developed copiers, problems as mentioned below come up.
Firstly the electrophotographic OHP sheet disclosed in Japanese Tokkai Hei 5-289390 (the term xe2x80x9cTokkaixe2x80x9d as used herein means an xe2x80x9cunexamined published patent applicationxe2x80x9d) is taken as an example, which has a toner-receiving layer comprising polyester resin having a glass transition temperature of 40-75xc2x0 C. and a molecular weight of 1,000 to 20,000 and a matting agent having a particle size equal to or greater than the thickness of the toner-receiving layer.
When the recording on such a sheet is carried out at a low speed with a conventional type of electrophotographic copier having a large supply of oil, the sheet can ensure good running ability and recorded images of high transparency. In the case of high-speed recording, however, the sheet has a problem that the images recorded thereon are inferior in color reproducibility and transparency because it is difficult to soften the toner-receiving layer in an instant by heat applied for toner fixation and so the toner cannot be buried instantaneously into the toner-receiving layer.
In the case of recording with an electrophotographic copier of the type which is reduced in oil supply, on the other hand, there occurs a problem that the foregoing OHP sheet is apt to entwine itself about the fixing roller and so the transportability thereof becomes poor.
In addition, the electrophotographic OHP sheet having a toner-receiving layer constituted of a thermoplastic resin, such as polyester resin, styrene-acrylic resin or epoxy resin, and a plasticizer containing at least one per molecule of aromatic ring or a polyalkyleneoxide polyol plasticizer is disclosed in JP-A-9-152736. This reference further discloses that the suitable softening point of the resin is from 40 to 180xc2x0 C., preferably from 90 to 140xc2x0 C.
The plasticizer added to the toner-receiving layer of the electrophotographic OHP sheet has an effect of increasing the fluidity of the toner-receiving layer upon application of fixing heat, and thereby the color reproducibility and transparency of recorded images can be enhanced so far as the recording is carried out with a conventional type of electrophotographic copier having a full supply of oil. In the case of using an oil supply-reduced electrophotographic copier, however, such an OHP sheet has problems that it still has a tendency to entwine itself about the fixing roller and the images recorded thereon at a high fixing speed are inferior in color reproducibility and transparency. Further, the compatibility between the thermoplastic resin and the plasticizer is poor, and so the plasticizer is liable to bleed with a lapse of time. As a result, the foregoing OHP sheet has a problem that the efficiency in winding for finishing it as a product is low and the keeping quality in process of being distributed as a product is inferior (to cause a blocking trouble).
In other words, every OHP sheet according to the prior arts fails to achieve good travelling properties and high-speed recording suitability as well as to have no oily feel when it is applied to an electrophotographic copier having a reduced oil supply.
Therefore, the present invention is made for solving the aforementioned problems. In other words, an object of the invention is to provide an electrophotographic OHP sheet which not only ensures no oily feel on the recorded surface but also has excellent travelling properties and high-speed recording suitability when the recording is performed thereon by means of an electrophotographic copier of the type which requires no oil supply to the fixing roller or can accomplish an oil supply to the fixing roller by the use of non-woven cloth impregnated with a slight amount of oil, or by means of a color laser printer. Additionally, electrophotographic OHP sheets according to the invention are intended to also have applicability to conventional type of electrophotographic copiers wherein a large quantity of oil is supplied to the fixing rollers.
The foregoing object of the present invention is attained with an electrophotographic OHP sheet comprising a transparent support and a toner-receiving layer formed on one side of the support, characterized in that the toner-receiving layer has suitability for toner fixation at a temperature in the neighborhood of 160xc2x0 C. to 200xc2x0 C. and comprises (a) a resin having a softening point of from 180xc2x0 C. to about 190xc2x0 C., (b) a resin having a softening point of from 110xc2x0 C. to 140xc2x0 C. and (c) a plasticizer having a plasticizing efficiency of 1.1 or below.
By further adding to the toner-receiving layer (d) a matting agent the grain size of which is in the range of 3 to 20 xcexcm and at least 1.2 times the thickness of the toner-receiving layer, the travelling properties, in particular, of the present electrophotographic OHP sheet can be improved. Furthermore, the present electrophotographic OHP sheet can have thereon highly transparent recorded images, in particular, by using a polyester plasticizer as the plasticizer (c).